This invention relates to a sensor element for detecting trace amounts of gaseous oxidizable substances such as alcohols, aldehydes, hydrocarbons, carboxylic acids, amines, carbon monoxide, and hydrogen contained in the atmosphere, exhaust gases and the breath and a method for detecting the oxidizable gas.
For detecting the above-noted gaseous substances in the atmosphere, an exhaust gas, the breath, etc., there have hitherto been known various methods such as gas chromatography, chemical analysis, and nondispersive infrared absorption spectroscopy. These detecting methods, however, have such disadvantages as complexity of the device, requirement of skill for the analytical procedure, lack of instantaneousness owing to the time-consuming procedure, unsuitableness for a sample gas rapidly changing in composition owing to long intervals between samplings, and expensiveness of the device.
On the other hand, among devices which make use of a semiconductor as the sensing element, there has been known an ethanol sensor comprising n-type tin oxide. This element is evaluatd as having been improved to some degree in the above-said disadvantages. However, when the said substance is used in detecting ethanol, the ethanol is adsorbed on the semiconductor surface. Consequently, although the element is effective for the first sensing operation, it is unsuitable for a continuous use. In case it is to be used repeatedly, it must be heated each time at a temperature of 350.degree. C. or higher to desorb the ethanol. The element has additional disadvantages in that it is incapable of quantitative sensing because of failure in responding proportionally to the ethanol concentration and that in order to compensate a large temperature coefficient of its electric resistance, the external circuit connecting to the sensing element becomes complicated.